US2994876A - Ultrashortwave antenna - Google Patents

Description

1961 B. A. s. JOSEPHSON 2,994,876

ULTRASHORTWAVE ANTENNA Filed Jan. 9, 1958 4 Sheets-Sheet l PRIOR "3] d FIG1 FIG 2 FIG? FIG 5 inventor:

Aug. 1, 1961 B- A. s. JOSEPHSON 2,994,876

ULTRASHORTWAVE ANTENNA Filed Jan. 9, 1958 4 Sheets-Sheet 2 V1 If V Zb FIG 6 FIG 7 kI (X)I (X) 1 (2k)I (0)+I (0) FIG 8 FIG 9 Inventor:

g- 1961 B. A. s. JOSEPHSON 2,994,876

ULTRASHORTWAVE ANTENNA 4 Sheets-Sheet 3 Filed Jan. 9, 1958 FIG 18 FIG '10 FIG 12 FIG 11 Inventor:

Aug- 1, 1961 B. A. s. JOSEPHSON 2,994,876

ULTRASHORTWAVE ANTENNA 4 Sheets-Sheet 4 Filed Jan. 9, 1958 FIG '13 FIG 15 FIG 16 United States Patent i 2,994,876 ULTRASHORTWAVE ANTENNA Bengt Adolf Samuel Josephson, Rasundavagen 68, Solna, Sweden Filed Jan. 9, 1958, Ser. No. 709,432 Claims priority, application Sweden Jan. 14, 19 57 12 Claims. (Cl. 343-803) The antenna according to this invention is suitable for use at short radio waves. It can be constructed of elements in the form of wires, tubes or plates, or of a stripline system. Its radiation pattern is essentially the same as that of a rod antenna, i.e., it is essentially omnidirectional in a plane perpendicular to its length direction. By combining two or more driven or parasitic antenna elements designed according to the invention, directional properties can be achieved.

A certain embodiment of the invention is particularly suited for use as an external antenna on an aircraft.

In another embodiment it is suitable as receiving antenna for VHF and UHF television and broadcast receivers.

On modern aircraft the antenna problem becomes more and more difiicult to solve, due to the increasing number of antennas needed, the severe requirements on their mechanical strength and streamlined shape at high speeds, the claims for an accurate match to the feeding line within large frequency bands, and so forth. The use of external antennas on an aircraft has many drawbacks, such as increased drag, influence on the stability of the airplane, risk of ice formation and risk of damage. The use of suppressed antennas and slot antennas would solve some of these difficulties, however, this is often not feasible, because the necessary space within the skin of the aircraft might not be available.

Accordingly it is very desirable that appropriate external antennas be devised, and above all one must observe, that these get the least possible height, good streamlined shape, good broadband impedance match and that they be of simple and inexpensive design in view of the need for exchange when damaged.

The antenna according to the invention fulfills these requirements.

In the embodiment of the invention, which is intended for aircraft, the radiating elements may consist of the wider conductors of a strip-line system, where the conductors have the form of very thin metal foils of different width, directly applied to thin dielectric plates.

An antenna constructed of such plane conductors, which have the double function of radiating elements and impedance correcting elements for the accomplishment of a good match to the external feeding line, is in the following called a strip-line antenna.

Antennas for VHF and UHF television and broadcast receivers consist generally of single or folded half-wave elements or of arrays of such elements. At channel 4, for instance, the elements must have a length of about 230 cm. With regard to the mechanical support and the space requirement it would be of considerable advantage if such elements could be made shorter.

The achievement of such shortening is one of the objects of this invention.

The invention is in the following explained with the aid of a number of illustrations.

FIGURE 1 shows an ordinary folded quarter-wave unipole (half a folded dipole), FIGURE 2 shows an open folded unipole, FIGURE 3 shows an open folded unipole with displaced feed-point according to the invention, FIG- URE 4 shows an open folded dipole and FIGURE 5 shows an open folded dipole with displaced feed-point.

In FIGURES 6-9 these different arrangements of fold- 2,994,876 Patented Aug. 1, 1961 ice ed unipoles and equivalent circuits are shown for explaining the theory of the invention.

FIGURE 10 shows an embodiment of the invention which is especially short, FIGURES 11 and 12 show an 5 open folded unipole and dipole, respectively based on unshielded two-wire lines.

FIGURES 13-15 show three projections of an embodiment, which is particularly intended for use on an air craft, and FIGURES 16-17 show how the same can be built into a plastic covering of suitable aerodynamic shape.

FIGURE 18 finally shows how a shortened antenna according to the invention can be equipped with shortened parasitic elements for increasing the directivity.

The conventional antenna in in FIGURE 1 has a real impedance at a length of about M4, A being the wavelength. The antenna according to FIGURE 2, in which the folded arm is open (isolated from ground), has quite different impedance properties from the conventional folded unipole. It has a real impedance at a length of about 0.15 thus being about 40% shorter than a quarter-wave. However, this antenna in most cases is not very useful, because its radiation resistance is very low, of the order of 10 ohms.

With the antenna according to FIGURE 3 the advantage of a small length is maintained, while the drawback of low radiation resistance is avoided. By suitable dimensioning (choice of the parameters x, a and d) the antenna can be accurately matched to a normal feeding cable at a wavelength of about 7 's.

The folded unipoles above a conducting plane according to the previous figures may be completed to free dipoles as shown in FIGURES 4 and 5. The impedance will then be doubled.

The impedance properties of these new antenna types are derived in the following manner.

A folded unipole which is loaded with an impedance Z, (FIGURE 6) and driven with a voltage V, whereby the voltage V is developed across the load impedance, is equivalent to the circuit in FIGURE 7.

The total current -I in the antenna is composed of the balanced current LE from the sources 2 and the unbalanced current 21 from the source 1. This latter current may be arbitrarily divided between the two branches, with the amount (2-k)I in the driven arm and H in the other.

If the antenna is symmetrical, seen from source 1, then k=l. If the antenna consists of two conductors with different cross sections, then k may be computed from the dimensions.

The admittance of the antenna is The currents are obtained from the relations where From Equations 1-4 one obtains the antenna admittance Y granule-brawn] (5) kYa+ a+ b For the open folded unipole according to FIGURE 2 one has Y =0. With rewriting of Formula in impedance form one then gets characteristic impedance of the two antenna conductors regarded as a twin-line, will give If s 7\/4, then Z has a negative and the second term a positive reactance. At a certain length s \/4, Z is real. For Z =300 ohms and k=1 (symmetrical antenna) this occurs at s-0.15- as mentioned previously.

If the driven arm is made thinner than the open, k 1 and thus 2,, is real at a still shorter length.

The radiation resistance Re{Z is in these cases about ohms or slightly less.

It shall now be shown, that the special arrangement according to FIGURE 3 yields a higher value of the radiation resistance, suitable for matching the antenna to a feeding cable with normal value on characteristic impedance (50 ohms or 75 ohms). For the sake of simplicity we shall first assume that the antenna conductors be sufficiently thin that the currents, the balanced current I as well as the unbalanced current I may be regarded as sinusoidally distributed along the conductors.

If x is the distance from the ground plane, the balanced current as a function of x is cos ,B(s:v) 1.00 2(0) and the unbalanced current sin 18s simplifications 2 RX: (srn 26s) sin [32:

Around those frequencies where the antenna impedance is real, the simple Formula 11 is in good agreement with measurements on fairly thin antennas according to the invention, and even in the case of thick antennas the formula qualitatively describes the behaviour.

As an example of the principles shown it can be mentioned, that a a diameter 2a=3.5 mm., the length s=22 cm. on the antenna conductors, the impedance Z =12O in d/a=300 ohms and x=10 cm. (see FIGURE 3), a real antenna impedance of 50 ohms is obtained at the wavelength 145 cm., i.e. at s/l\=O.l5.

For an explanation of the operation of the FIGURES 5 and 12 embodiments, consider now the case of the schematically drawn antenna in FIGURE 9 taken together with its image in the ground plane. One obtains an open dipole, having two symmetrically located voltage sources of equal magnitude and such direction that the currents from the two sources have same direction'at the junction between the two antenna halves. Compared with the corresponding unipole this antenna has twice the length and the same impedance at each driving point, or twice the impedance, if based on the sum of the voltages at the two sources, in the manner shown in FIGURE 12. However, it is not necessary to drive the antenna at two points. Essentially the same result-a real impedance of suitable value at essentially same antenna lengthcan be achieved if the antenna is driven in one point only, as shown in FIGURE 5, demonstrating a case, where the antenna is built-up of coaxial lines and driven from a coaxial feeder.

If the antenna be so dimensioned that the parameter k in the formulas is larger than 1, the antenna impedance will be real at a still shorter length, as may be seen from Equation 6a. This is done mechanically by making the diameter of the right branch in FIGURES 2 or 3 bigger than the diameter of the left branch, whereby the unbalanced current 21 (FIGURE 7) is divided with a bigger amount on the thicker conductor than on the thinner. The same effect can be achieved by enlarging the electrical cross section of the appropriate conductors in the way illustrated in FIGURE 10. -'By this arrangement the antenna can be matched to the feeding cable at a length 2s=7\/ 4, or even less, and this antenna may, therefore, be named a quarter-wave dipole.

In the foregoing examples on the principle of the invention to shorten and match an antenna, coaxial lines have been contemplated. However, screened or unscreened two-wire lines may be used as well. In FIG- URE 11 it is shown, how this can be accomplished in the case of a unipole and in FIGURE 12 for a symmetrically driven dipole. In these cases the antenna conductors may consist of a 300 ohm twin cable of conventional design, i.e. of two solid or stranded conductors in a polyethylene tape. Mechanical support and protection against atmospheric influence can be achieved by inserting the antenna into a plastic tube.

FIGURES 13-15 disclose still another embodiment of the invention, which is intended especially for use at an aircraft the antenna thus being of the aforenamed stripline type comprising thin dielectric plates, whereupon metal foils are intimately applied. Here it has been possibleto combine all the features characteristic for a strip-line antenna, such as simple mechanical design, good aerodynamical properties, inherent possibilities of built-in feeding and impedance compensating elements, and large bandwidth, with the exceptional low height compared with the wavelengths at which the antenna is matched, which characterizes the new unidipole or dipole antenna principle.

The narrow conductor 1 forms, in conjunction with the insides of the wide conductors 2 and 3 connected to the counterpoise, a built-in feeding line, driving the antenna in the point P between the conductors 2 and 3 in parallel and the upper end of the conductor 4, which is positioned between the free ends of conductors 23 and the counter-poise, thus the conductor 4 from the feeding point being directed down towards the counter-poise. The radiating parts of the antenna are the conductor 4 and the outsides of the folded conductors 2 and 3. The antenna conductors consist, as said above, of metal foils, which are applied to dielectric plates 5 and 6.

FIGURES 16 and 17 illustrate how the antenna can be embedded in a plastic covering in order to yield increased mechanical strength and proper aerodynamical shape.

In a similar way as for driven antenna elements, parasitic elements (directors and reflectors) can be shortened. FIGURE 18 shows a three-element Yagi antenna, in which the fed element is designed according to the invention. All three elements have a length of approximately 0.3 wavelength. The exact length of the passive elements, as for ordinary Yagi antennas, must be determined in such a way, that their reactances give the induced currents a proper phase with regard to co-operation in the forward direction with the current in the driven element, respectively counteracting in the backward direction.

What is claimed is:

1. Folded antenna, comprising at least one conducting U-shaped member having arms of unequal length, at least one auxiliary conducting member spaced apart from and extending substantially in the longitudinal direction of the shorter arm of said U-shaped member and spaced apart transversely thereof substantially less than a quarter of a wavelength at the lowest operating frequency from the longer arm of said U-shaped member, means for maintaining the potential constant at the extremity of the longer arm of said U-shaped member, and means for driving the antenna between adjacent extremities of said shorter arm and said auxiliary member, the other extremity of said auxiliary member being isolated.

2. An antenna in accordance with claim 1, in which the longer arm of said U-shaped conducting member is substantially shorter than a quarter of a wavelength at the lowest operating frequency.

3. An antenna in accordance with claim 1, comprising an additional conducting U-shaped member, placed opposite to said first U-shaped member and having one arm integral with the longer arm of said first U-shaped member.

4. Symmetrical folded dipole antenna, wherein each half is equal to a folded unipole antenna in accordance with claim 1, thus comprising a symmetrical conducting member U-shaped at both ends, two auxiliary, equiform conducting members symmetrically positioned with respect to the plane of symmetry, and means for driving the antenna simultaneously across two symmetrically positioned gaps.

5. An antenna in accordance with claim 4, comprising driving means in the form of two-built-in feeding lines, and a connecting means disposed at the plane of symmetry for connecting an external feeding cable, each of said feeding lines including a first narrow conductor, which runs from said connecting means along one half of said symmetrical conducting member to one of said auxiliary members, the second conductor of same feeding line being constituted by same half of the symmetrical member.

6. An antenna in accordance with claim 1, in which means are provided for connecting the extremity of said longer arm of said U-shaped member to a counter-poise.

7. An antenna in accordance with claim 6, comprising driving means in the form of a built-in feeding line, and a connecting means disposed at the junction between said U-shaped member and the counter-poise for connecting an external feeding cable, said feeding line including a first, narrow conductor which runs from. said connecting means along said U-shaped member to said auxiliary member, the second conductor of said feeding line being constituted by said U-shaped member.

8. An antenna in accordance with claim 7, in which said U-shaped member consists of a metal tube, which encloses said first conductor of said feeding line.

9. An antenna in accordance with claim 7, in which said U-shaped member and said first conductor of said feeding line consist of the two conductors of an unshielded, properly bent and cut two-wire cable.

10. Folded antenna, comprising at least one conducting U-shaped member having arms of unequal length, at least one auxiliary conducting member spaced apart from and extending substantially in the longitudinal direction of the shorter arm of said U-shaped member, means for driving the antenna betwen adjacent extremities of said shorter arm and said auxiliary member, and at least one thin substantially plane dielectric plate, said U-shaped and auxiliary conducting members consisting of metal foils having a large width compared to the thickness of said dielectric plates, said metal foils being applied to the surfaces of said dielectric plates.

11. Directional antenna, comprising at least one driven element consistent with claim 1, and at least one parasitic conducting U-shaped element.

12. Antenna as claimed in claim 10, in which the structure comprising the dielectric plates and metal foils is embedded in a streamline shaped dielectric covering.

References Cited in the file of this patent UNITED STATES PATENTS 2,285,669 Lehmann June 9, 1942 2,372,651 Alford et al. Apr. 3, 1945 2,512,078 Wehner June 20, 1950 2,614,219 Cary Oct. 14, 1952 2,647,211 Smeby July 28, 1953 2,725,473 Darling Nov. 29, 1955 2,785,396 Carter Mar. 12, 1957 FOREIGN PATENTS 692,692 Great Britain June 10, 1953 703,705 Great Britain Feb. 10, 1954

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